1. Field of the Invention
This invention relates to polyamide resins from dimer acids and their use in thermographic inks. The polyamide resins are prepared from an acid component comprising certain dimer acids and monomeric fatty acids, and an amine component comprising a mixture of a diamine, such as ethylene diamine and a polyalkylene polyamine, such as diethylene triamine.
2. Description of Related Art
Polyamide resins derived from polymeric fat acids have been known for a long period of time and have been developed commercially. Specific resins have been developed and adapted for a variety of uses including hot melt adhesives, coatings, films and inks particularly flexographic inks. Such resins have been prepared employing, in addition to the polymeric fat acids, a variety of copolymerizing dicarboxylic acids, monomeric monocarboxylic acids and a variety of amines. Flexographic inks employing polyamide resins prepared from dimer acids, mixtures of diethylene triamine and ethylene diamine, and lower aliphatic monocarboxylic acids such as acetic acid, are described in U.S. Pat. No. 3,253,940. CAS Registry Number 68475-87-6 describes polymers of dimerized C.sub.18 unsaturated fatty acids with azelaic acid, isostearic acid, diethylene triamine and ethylene diamine.
A resin from tall oil acid dimer, hexamethylene diamine, azelaic acid and ethylene diamine was developed for use in thermographic inks. While having many desirable properties for such use, including an initially glossy smooth surface, such product over time develops a dull haze, referred to in the art as a bloom or blush, which significantly detracts from the commercial use of the resin as a thermographic ink. In the thermographic process a raised print is produced by putting down a layer of ink on paper, coating the ink with a thermographic powder and fusing the powder with heat to give a smooth, glossy surface. Because of bloom, the typical tall oil fatty acid based dimer resins, have found limited use in thermographic ink processes for small printing area applications, such as business cards and stationery, and found little application in large printing areas, such as packaging and greeting cards.